1. Field of the Invention
This invention is for an overhead projector to be used with at least one large stage aperture and a small stage aperture above a Fresnel lens, and with a light source mounted below the plane of the stage, characterized by a single or multi-element condenser which can be positioned between the lamp housing and Fresnel lens. In the case of a small stage aperture, the condenser focuses the light flux designed for the large stage aperture onto the small stage aperture.
2. Description of the Prior Art
Generally, the stage aperture of an overhead projector is matched to the DIN A4 format, which has a size of about 285.times.285 mm. Consequently, the illumination is also designed for these dimensions. The reflector lamp and any following condenser, and the Fresnel lens mounted below the operational surface, are so matched to the 285.times.285 mm dimensions that the maximum possible light flux passes into the operational surface and the projection lens.
If, however, less than a full stage aperture shall be used for projection, then the light flux emitted by the lamp shall be needlessly spread over the entire surface of the stage. For example, with LCD (liquid crystal displays) overhead displays, the image screen data of a data-processing apparatus is transmitted by an overhead projector onto a projection wall and thereby is made visible to a substantial number of viewers. The color displays in particular are quite dense and therefore require a highly efficient light flux so that data details can be recognized on the projection wall. It would make sense to guide the available luminous light flux only through this lesser stage aperture in order to substantially increase the luminous light intensity there. As in the case of the LCD, this is especially advantageous when the artwork on this smaller stage aperture is denser than the artwork transparency ordinarily used on the larger stage aperture.